The present invention relates to a transformer which shields electrical noises, more especially to a noise-shielded transformer which suppresses electrical noises by absorbing conductible electrical noises that flow into the power line to cause interference. A noise-generating device becomes a noise source to other peripheral electronic devices and the electrical noises from such a device, which is one of the power line disturbance, is regulated as an electromagnetic interference since the electrical noises have a bad effect on other peripheral electronic devices.
Electronic devices, which are not noise-generating sources, is regulated by electromagnetic susceptibility since such electronic devices are subject to a software malfunction or a hardware breakdown due to external electrical noises.
Accordingly, measures against the electrical noises are required for reliability enhancement and lifetime protection of various electronic devives. That is, a noise-shielded transformer is required which can prevent electrical noises generated by a noise-making device from flowing into other peripheral devices in order to protect them and also protect devices used as loads against external noises.
FIG. 3 is a plain view showing a conventional noise-shielded transformer. As shown in FIG. 3, the conventional noise-shielded transformer has a configuration that a predetermined number of turns of wire is wound around both the left and the right of the cut-core or the U-I iron core 1 for the primary winding 2 and the secondary winding 3, these windings of the primary 2 and the secondary 3 are covered with insulators 4a, 4b for an electrical insulation, conductive plates 5a, 5b are wrapped over the insulators 4a, 4b, conductive plates 6, 7 are mounted amidst said iron core 1 in order to magnetically isolate said primary winding 2 and secondary winding 3, the terminals of said primary winding 2 are connected to an alternate power source, and the terminals of said secondary winding 3 are connected to a load 8.
In the conventional noise-shieled transformer having such a configuration, when an alternate power source including a pulse-property noise flows into the primary winding 2, the pulse-property noise generates magnetic flux of a high frequency with flowing in the primary winding 2, wherein the iron core 1 minimizes the magnetic flux of a high frequency generated by the pulse-property noise since the iron core 1 made of the material sharply decreasing the magnetic permeability over high frequencies is used for low frequencies.
Accordingly, a noise induction in the secondary winding 3 is minimized since the useful magnetic flux caused by the high frequency noise flowing into the primary winding 2 is minimized.
In the meantime, the conductive plates 5a, 5b prevent a noise induction from being generated by the influence of external electric and magnetic fields on the primary and secondary windings 2, 3, the radiation of a high frequency noise flowing into the primary winding 2, and the noises conducted past the capacitance between the primary and secondary windings 2, 3 since the conductive plates 5a, 5b are grounded to reduce the capacitance between the primary and secondary windings 2, 3 and by-pass the noises to the ground. And also, the conductive plates 6, 7 magnetically isolate the primary and secondary windings 2, 3 in order for their interference to be blocked, and minimize the capacitance between the primary and secondary windings 2, 3.
In particular, the conductive plates 5a, 5b suppress noises because the plates generate a large reverse excitation power against a high frequency noise to suppress the useful magnetic flux due to the noise just like the case that the magnetic characteristic change of the iron core 1 is used. However, such a conventional transformer has drawbacks in that its productivity is low since its insulating process and assembly are difficult by using the cut-core or U-I core when constructing its magnetic circuit, its magnetic circuit is apt to have a bad effect on other devices because a very high leakage of magnetic field is made by its configuration, its voltage change rate which is one important characteristic of a transformer goes very bad, its noise eliminating capability becomes bad by the effect of the leakage of the magnetic flux over a range of frequency beyond 100 KHz, a smaller and lighter transformer can not be made because the configuration of its magnetic circuits makes a high magnetic flux design impossibe, and its efficiency is lowered.